Is Hydrogen the fuel of the future?

The idea was appealing. Ties to foreign oil fields would be severed, and nothing but water vapour would emerge from such a vehicle's exhaust pipe. Congress duly approved the money, and the Department of Energy and other research agencies got to work. But then the whole effort faded into obscurity, as attention shifted first to biofuels and then to battery-powered electric vehicles. Both seemed to offer much quicker and cheaper routes to low-carbon transportation.

The shift seemed complete when the US Secretary of Energy Steven Chu entered office last year. Chu outlined four primary pitfalls with the hydrogen initiative. Car manufacturers still needed a fuel cell that was sturdy, durable and cheap, as well as a way to store enough hydrogen on board to allow for long-distance travel. Hydrogen also required a new distribution infrastructure, and even then the greenhouse-gas benefits would be marginal until someone worked out a cost-effective way to make hydrogen from low-carbon energy sources rather than natural gas.

May 1990, four months after being sworn in, Chu announced that the government would cut research into fuel-cell vehicles. Biofuels and batteries, he said, are "a much better place to put our money." Chu’s statement served only to energize the supporters of hydrogen vehicles, and it became clear during subsequent months that the debate was far from over.

On 9 September in Stuttgart, Germany, nine major car manufacturers — Daimler, Ford, General Motors, Honda, Hyundai, Kia, Renault, Nissan and Toyota — signed a joint statement suggesting that fuel-cell vehicles could hit dealerships by 2015. In a coordinated announcement in Berlin, a group of energy companies including Shell and the Swedish firm Vattenfall joined Daimler in an agreement to begin setting up the necessary hydrogen infrastructure in Germany.

This push for rapid deployment has left many people shaking their heads. "I just don't see it," says Don Hillebrand, director of the Center for Transportation Research at the Argonne National Laboratory in Illinois. "It doesn't make sense."

Yet the proponents of hydrogen vehicles are brimming with confidence. "This memorandum of understanding marks the will of the industry to move forward," says Klaus Bonhoff, who heads the National Organisation for Hydrogen and Fuel Cell Technology (NOW), a Berlin-based organization created by the German government in 2008 to spearhead that country's hydrogen programme.

Nature assesses four major challenges facing hydrogen fuel-cell vehicles, and finds that some of the challenges are close to being met — but others have a long way to go.

Fuel cell

A fuel cell is simply a device that takes in oxygen from the air and hydrogen from a tank, and reacts them in a controlled way to produce water vapour and electric power. In a vehicle, that power can then be directed through an ordinary electric motor to turn the wheels.

In practice, fuel cells are anything but simple: controlling the reaction and extracting the electric current requires a sophisticated assembly including nozzles, membranes and catalysts. And therein lies the challenge: how to pack all that complexity into a device that is light, cheap, robust and durable — as well as being powerful enough to provide rapid acceleration, plus drive all the lights, air conditioning, radio and other amenities that consumers have come to expect in a modern vehicle.

Ten years ago this goal seemed far off. Car manufacturers didn't even dare to expose their experimental fuel-cell vehicles to cold weather: they worried that when the cells shut down, residual water vapour could freeze and wreak havoc on the delicate insides. Instead, the companies would shuttle the vehicles around in heated trailers.

But a decade has brought fuel-cell technology a remarkably long way. Byron McCormick, who headed the fuel-cell programme of General Motors until January 2009 affirms "It has really been a whole lot of small steps."

For example, General Motors' fuel-cell vehicles eliminate the cold-weather problem in part by continuing to run the cell's exhaust system for a minute or two after the car is shut down, using the cell's residual heat to drive the water out of the system. Toyota says that its experimental, fuel-cell-equipped Highlander sports-utility vehicle will start up at −37 °C.

Engineers are also cutting back on the use of expensive catalysts. General Motors' fuel-cell assembly uses roughly 80 grams of platinum to split electrons and protons from hydrogen atoms. But General Motors officials say that their next fuel cell will use less than 30 grams of platinum, thanks to using ever thinner coats of the metal. And the company's scientists are continuing to experiment with measures such as increasing the surface area of the catalyst by introducing more texture at the nanoscale. Within a decade, they expect to get platinum use to below 10 grams, which would make the fuel cells competitive with today's catalytic converters in terms of precious-metal use.

These and other advances translate into price reductions. The Department of Energy estimates that fuel-cell costs per kilowatt of power dropped by nearly 75% between 2002 and 2008, based on cost projections for high-volume manufacturing. Companies won't discuss retail prices except to say that the vehicles slated to appear by the middle of the decade will be priced competitively. "I've been doing this for 10 years, and the numbers even surprise and shock me," says Craig Scott, manager of Toyota's advanced technologies group in Torrance, California. "It is definitely going to be a car that is in reach of a lot of people."

On-board storage

In June 2009, Toyota engineers and US government monitors hopped into a pair of fuel-cell Highlanders in Torrance and took a 533-kilometre round trip through real-world traffic — without refuelling. Calculations suggest that the vehicles' performances corresponded to a range of 693 kilometres on a single tank of hydrogen, which is on a par with the range of current petrol vehicles.

Gaseous hydrogen is easy enough to store in a tank. But getting enough of it on board would require either a ridiculously large tank or an exceptionally strong tank that could safely store compressed hydrogen gas at hundreds of times atmospheric pressure. Liquid hydrogen is much denser, but it would have to be maintained in an insulated tank at −253 °C.

In the end, the comparative simplicity of compressed hydrogen won out. Most companies have chosen to use modern carbon-fibre tanks, which can store hydrogen at up to 680 atmospheres, while still being relatively lightweight. To improve range further, many companies are equipping their vehicles with the same 'regenerative braking' technology that allows hybrid petrol and electric cars and all-electric cars to capture energy during braking, store it in auxiliary batteries, and reuse later.

Indeed, because hydrogen and battery-powered vehicles both use electric motors, they share many technologies. The only real difference is the power source: fuel cells versus batteries. Scott says that electric vehicles based on the lithium-ion battery chemistry are unlikely to get beyond a range of 150–250 kilometres on a single charge. Although that may be enough to cover urban driving, consumers like having the option to drive cross-country. So in the shift away from petrol, the hydrogen vehicle's greater range could give it an edge in the long term.